Optical force measuring elements provide the advantage over other force measuring elements such as those based on DMS in that optical force measuring elements are free of electrical lines and connections. This is particularly advantageous if they are used in environments where electromagnetic disturbances can be expected. Furthermore, the possibility of miniaturization is limited for such measuring elements based on DMS because of the electrical contacts. Optical force measuring elements are useful for medical purposes, especially for invasive applications. Other fields of use include for example robotics technology. The differentials between the optically measured distances are proportional to the acting forces that deform the structure.
WO 2007/015139 describes a catheter with a sensor tip having an optical force measuring element attached to the end thereof. This sensor tip can be placed in a human organ, for example in a heart, at the wall thereof where it determines the three-dimensional force acting on the sensor tip that is generated by the organ wall. In this way, a so called “mapping” of vessels and organs can be performed.
The publication by J. Peirs, J. Clijnen, P. Herijgers, D. Reynaerts, H. Van Brussel, B. Corteville and S. Boone “Design of an Optical Force Sensor for Force Feedback during Minimally Invasive Robotic Surgery” describes an optical sensor which can be used for example for the application mentioned above. It describes an elastic cylindrical outer structure in the center of which three optical fibers with three fiber ends have been axially inserted. These measure the distance to a fixed end which approaches the fiber ends depending on the action of a force onto the upper part of the structure. A disadvantage of this sensor is the limited possibility of miniaturization. In addition, the measurements of the forces in the three x, y, and z directions are always linked to each other.
US 2008/0009750 suggests a catheter tip incorporating the structure mentioned in the beginning. It is identical to the structure already described in the publication by Peirs et al. It consists of a tube cut into a flexible structure by radially disposed notches in the central region. The flexible structure has a plurality of columns that are alternatingly connected to an upper and a lower annulus wherein each adjacent pair of columns is connected to each other by two flexible bridges. In the lower annulus, optical fibers are arranged in openings that can measure the distance to the lower ends of the columns attached at the upper end or to the bridges to thereby deduce the forces acting on the upper annulus.
Since all notches and thus all edges of the columns and the elastic bridges extend radially with respect to the tube axis, the columns at the tube outer wall are thicker than those at the tube inner wall, and the bridges are softer at the tube outer wall than those at the tube inner wall because the latter are shorter. For this reason, the tube must be provided with a thin wall thickness with respect to the tube outer diameter, about 1:10, to ensure flexibility of the elastic bridges. The dimensions mentioned of 0.5 mm wall thickness and 5 mm tube diameter must not be under-run because otherwise the structure would become too unstable. However, for many applications, for example also for catheters, smaller sensors are demanded.
Another disadvantage of this structure is that due to the spiral construction of the structure, a force acting both in the axial and in the radial direction onto the structure always causes torsion. This torsion as a consequence of the structure makes it difficult to calculate the three-dimensionally acting forces from the distance measurements obtained. In particular, it is not obvious in the catheter described as to how a force acting on the catheter tip is transmitted to the tube since no connection to the catheter tip is provided except via the outside wall. Therefore, an axial load would scarcely be transmitted to the measuring element.